1. Field of the Invention
This invention relates to an optical stop device of the type arranged to adjust the quantity of light passing through an optical system, and more particularly to a stop device for a camera or the like having a physical property member which is arranged to have its light transmission factor varied in response to an electric or magnetic field impressed thereon.
2. Description of the Prior Art
In the past, a mechanical stop device has generally been disposed in the optical system of a camera for adjustment of light quantity. A mechanical stop device necessitates the use of a mechanical mechanism for its operation. Particularly, in a camera adapted for use with an interchangeable lens, a mechanical stop device necessitates a complex coupling mechanism to provide a linkage between the stop mechanism of the interchangeable lens and the camera body. This necessity has caused an increase in the cost of such camera, an increase in its size, and has restricted the latitude allowed for the design of the camera.
However, use of an electrically or magnetically controllable stop device permits the complex stop control mechanism between the camera and the lens to be replaced with a simple arrangement to transmit signals through electrical contacts, and such a stop device is advantageous for a camera adapted for use with an interchangeable lens. Therefore, there have been proposed various stop devices of the type using an electro- or magneto-optical element that has a light transmission factor which changes with impression of an electric or magnetic field thereon. Known electro- or magneto-optical elements of this kind include Kerr cells utilizing a double refraction phenomenon, transparent strong dielectrics (PLZT), liquid crystals and Farady elements. It has been known to obtain devices for adjusting the quantity of transmitted light by combining such an element with a polarizer and an analyzer. In addition to these elements, there also are known electrochrome elements and elements that utilize the phenomenon of electrophoresis.
A physical property stop device using such an element has many advantages that cannot be found in the conventional mechanical stops. A salient advantage of such a device lies in the fact that it dispenses with a mechanical coupling or linkage mechanism. This permits reduction of size and weight and thus allows a greater latitude in designing a camera. Another advantage is that the absence of mechanical operating parts eliminates the shock and noise that are otherwise produced during a stopping operation. Therefore, the photographer is relieved of any disagreeable feeling. A stopping down action on the lens also can be readily confirmed by a switch operation. Besides, since a physical property stop is controllable with an electric circuit, it well meets the recent trend of applying electronics to cameras, and can be a great step toward the full application of electronics to cameras.
The aperture controlling methods of physical stops include an area control method in which the light transmission factor is controlled by varying the area of the aperture such as in a mechanical iris stop, and a light transmission factor controlling method in which the light transmission factor is controlled by varying an impression voltage or a magnetomotive force. In the latter method, the curve of the light transmission factor relative to the impressed voltage or the magnetomotive force, on the electro- or magneto-optical element, is too steep for fine adjustment of the light transmission factor. Besides, the light transmission factor varies with variation of temperature, etc. The latter method, therefore, is less practicable than the former. Further, in the latter case, it is impossible to utilize the variation in the depth of focus resulting from the effect of stopping. In view of these shortcomings of the latter method, the former, i.e. the area control method, has been mainly proposed for a physical property stop.
The stop of a lens which uses such a physical property member has a unique feature that is not found in the conventional mechanical stop. In other words, it is impossible with the capability of the current technology to make the maximum and minimum light transmission factors of the electro- or magneto-optical element 100% and 0% as in the case of the mechanical stop that uses an opaque material. In the presently known electro- or magneto-optical elements, the light transmission factor cannot be made to be 0% with electricity or magnetism impressed thereon to obtain a light shielding condition, because of a slight degree of light leakage and, in the light transmitting condition, a slight degree of attenuation of light is inevitable. These characteristics are inevitable because such light shielding and transmitting conditions are not brought about by moving an opaque material, but are brought about by merely utilizing the properties of the element in terms of solid- or liquid-state physics. In the use of a stop device for exposure control of a camera, however, this presents a serious problem.
Unlike the conventional mechanical stop which has a 0% light transmission factor in its light shielding part, a physical property stop of the area control type has a slight light leakage from its light shielding part as mentioned in the foregoing. Therefore, if the stopping area of the physical property stop are set in an equal multiplicative sequence in the same manner as in a mechanical stop, the effective T number (corresponding to the quantity of light received through the lens and the stop) at each stop aperture value would tend to deviate from the equal multiplicative sequence, and thus would become incompatible with the APEX computation system of the conventional camera. Then, in the case of a TTL camera of the type which measures light through a full-open lens and obtains shutter time information by introducing aperture information into a computation circuit, this incompatibility tends to cause an exposure error. To solve this problem, it is conceivable to combine a mechanical light shielding means with the physical property element, and to control the point of time at which the mechanical light shielding means operates in such a manner as disclosed, for example in DOLS 2541678. However, such a method not only requires the use of the mechanical light shielding means but also necessitates control over the point of time at which the light shielding means is to be operated. Such a method thus necessitates a complex control arrangement.